1. Field of the Invention
This invention pertains to bonded sputter target/backing plate assemblies and methods of making such assemblies.
2. Description of the Related Art
Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of a target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target materials to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated beneath or around a backing plate that is positioned in a heat exchange relationship with the target. The target forms a part of a cathode assembly that, together with an anode, is placed in an evacuated chamber filled with an inert gas, preferably argon. A high voltage electrical field is applied across the cathode and the anode. The inert gas is ionized by collision with electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, these ions dislodge the target material. The dislodged target material traverses the evacuated enclosure and deposits as a thin film on the desired substrate, which is normally located close to the anode.
In addition to the use of an electrical field, increasing sputtering rates have been achieved by the concurrent use of an arch-shaped magnetic field that is superimposed over the electrical field and formed in a closed loop configuration over the surface of the target. These methods are known as magnetron sputtering methods. The arch-shaped magnetic field traps electrons in an annular region adjacent to the target surface, thereby increasing the number of electron-gas atom collisions in the area to produce an increase in the number of positive gas ions in the region that strike the target to dislodge the target material. Accordingly, the target material becomes eroded in a generally annular section of the target face, known as the target raceway.
In a conventional target cathode assembly, the target is attached at a single bonding surface to a nonmagnetic backing plate to form a parallel interface in the assembly, as shown in FIG. 1. The backing plate is normally water-cooled to carry away the heat generated by the ion bombardment of the target. Magnets are typically arranged beneath the backing plate in well-defined positions in order to form the above-noted magnetic field in the form of a loop or tunnel extending around the exposed face of the target.
In order to achieve good thermal and electrical contact between the target and the backing plate, these members are commonly attached to each other by use of soldering, brazing, diffusion bonding, clamping, screwing or epoxy cements.
To a certain extent soft solders can accommodate stresses exerted on the target/backing plate assembly that occur upon cooling. Solder bonds of materials with widely differing thermal expansion rates, however, are susceptible to shear failure initiating at the extreme edges of the bond interface when the solder is too weak for the application. The result commonly experienced is debonding during service. To overcome the problem of joining one or more non-wettable materials by soldering, precoating with a metal is used to enhance solderability. These coatings may be applied by electroplating, sputtering or other conventional means. This need for intermediate coatings applied to target and backing plate materials that are difficult to wet and solder presents problems including adherence reliability of the applied coating and substantial added cost of applying the coating. Furthermore, the relatively low joining temperatures associated with the "soft" solders reduce the temperature range over which the target can be operated during sputtering.
The higher melting temperature solders used for high power applications are stronger but are far less forgiving of the stresses developed in the materials system. Targets of large size present greater stress problems as well as greater difficulty of producing sound bonds across the entire bond surface. As sputtering target sizes and power requirements increase, the soft solders become less applicable for joining of the material systems involved.
Another method which is applicable and used to some extent in target joining is that of explosive bonding or welding. By this technique, bonds are produced that combine solid state bonding and a mechanical interlocking as a result of the surface irregularities produced in the form of "jetting." The bonds are strong and reliable. The disruption of the initial mating surfaces during the dynamic bonding pulse negates the need for extreme surface cleanliness or preparation. See, e.g., John G. Banker et al., "Explosion Welding", ASM Handbook, Vol. 6, Welding, Brazing and Soldering; pp. 303-305 (1993).
Explosive bonding is a comparatively costly bonding method. For example, such bonding requires that the materials be provided in an oversized condition to allow for predictable damage at the periphery of the target assembly, thereby adding to material cost. Also, the conditions for achieving acceptable products must be adjusted for different component sizes and combinations of materials, and although the bonds offer good strength, the bond interfaces are variable in physical character. In addition, this method is not applicable to a materials system having one component which is brittle or which has limited ductility.
Smooth-surface diffusion bonding is an applicable method of bonding, but has only limited use in the bonding of sputtering target components. The bond is produced by pressing the material surfaces into intimate contact while applying heat, to induce metallurgical joining and diffusion to a varying extent across the bond interface. Bonding aids, metal combinations which are more readily joined, are sometimes applied to one or both of the surfaces to be bonded. Such coatings may be applied by electroplating, electroless plating, sputtering, vapor deposition or other usable techniques for depositing an adherent metallic film. It is also possible to incorporate a metallic foil between bonding members that has the ability to be more easily bonded to either of the materials to be joined. The surfaces to be joined are prepared by chemical or other means to remove oxides or their chemical films which interfere with bonding.
Smooth surface diffusion bonding requires extreme care in preparation and in maintaining surface cleanliness prior to and during the bonding operation to ensure reliable bond qualities. Because the diffusion bond interfaces are planar, they are subject to stressing in simple shear which commonly leads to peeling away at the ends of the bond area. The formation of brittle intermetallics at the bond interface, which increase in thickness with the associated long times of heat exposure, add to the potential of bond shear failure. An additional technique for bonding as described in U.S. Pat. No. 5,230,459 includes the pre-bonding step of providing machined grooves in the surface of one of the components to be solid-state bonded. This feature causes disruption of the bond surface of the associated component during heated pressure application. The material having the greater strength or hardness will normally be provided with the grooves such that, during bonding, it will penetrate into the softer member with the softer metal substantially filling the grooves.
Groove bonding is applicable to bonding many dissimilar materials, but is limited to materials that have dissimilar melting temperatures because the process must occur near the melting temperature of the lower melting point alloy. This precludes the use of this technique for similar metals. It is also possible that the saw tooth nature of the grooves may act as a stress concentrator and promote premature cracking in the alloys near the bonds. Furthermore, machining of the grooves is a time consuming operation.
In U.S. patent application Ser. No. 08/426,246, hereby incorporated by reference in its entirety, a method is disclosed for performing a surface roughening treatment to the bonding surface of the sputter target and/or backing plate, followed by solid state bonding. This roughening surface treatment provides 100% surface bonding compared to only 99% surface bonding in the absence of the surface treatment. The treatment further provides a bond with over twice the tensile strength of a bond formed from the non-treated smooth surfaces.
Irrespective of which above-described bonding method is used, conventional target cathode assemblies are limited with respect to the thickness of the target material that may be used to comply with the overall standard dimensions of the assembly as used and understood by the industry. The thickness of the target measured perpendicular to its sputter surface, plus the thickness of the backing plate, and the thickness of the backing plate at its periphery, measured in the same direction as that of the target, are set by the industry. Increasing the thickness of the sputter target would make the thickness of the overall assembly too large. Thin targets provide less material for sputtering, and must, therefore, be replaced frequently. Efforts to accomplish greater target thickness without altering the industry accepted dimensions have proved costly and ineffective. For example, the backing plate and target may be a one-piece construction made solely from target material. This allows more target material to be available for sputtering, which decreases the frequency with which the targets must be replaced. The target material, however, is generally expensive. It is thus preferable from a material cost standpoint to provide a two-piece construction with the backing plate made from a less expensive material.
Accordingly, it is an object of the invention to provide a convenient, inexpensive method for bonding thicker targets than have been previously possible to backing plates of either similar or dissimilar material where the bonded assembly is capable of withstanding thermal expansion and contraction stresses exerted thereon during and after sputtering.